In an optical communication system transmitting signals by sending a beam through an optical fiber, there has been used a beam intensity modulator capable of generating optical signals by modulating a laser beam emitted from a light source. The beam intensity modulator is formed by forming a Mach-Zehnder type optical waveguide, modulation electrodes, bias electrodes, and the like on a board made of electro-optic crystal such as lithium niobate (LiNbO3, hereinafter, abbreviated to LN).
The Mach-Zehnder type optical waveguide includes a division portion dividing an input beam, two arms permitting propagating the divided beams, and a beam combining portion combining the divided beams propagating through the arms. In the beam combining portion, when two merging beams are in-phase, the beam waves constitute an ON state in which the beam waves are output while intensifying each other. In contrast, when the two merging beams are out-phase, the beam waves constitute an OFF state in which the beam waves compensate for each other so that no light is output, A ratio of the intensity of the output beam in the ON state to the intensity of the output beam in the OFF state is called an extinction ratio and is an important index which shows a performance of the beam intensity modulator. In addition, as the extinction ration is higher, that is, as a difference between the intensities of the output beams in the ON state and the OFF state is larger, a modulation degree generally becomes larger. Accordingly, optical transmission of high quality is permitted.
Ideally, when output is zero in the OFF state, the extinction ratio becomes indefinite. However, in order to cause this situation, the intensities of the two merging beams have to be exactly equal to each other. However, the intensities of the merging beams may become asymmetric one another since a division ratio of the division portion is not the same due to a manufacture error of the optical waveguide or propagation loss in the two arms is different. In this case, there occurs a problem in that the extinction ratio deteriorates. That is because the two beams do not completely compensate for each other even though the two beams are out-phase.
As a method of making tee intensities of the beams symmetric in the beam collection portion to improve the extinction ratio, for example, a method of applying an excimer laser to the arm having a larger division power and permitting a defect in the optical waveguide to increase the propagation loss and balance with the intensity of the beam propagating trough the other arm can be taken into consideration. However, this method also has a problem in that the propagation loss made by permitting the defect depends on a wavelength and the extinction ratio also depends on a wavelength.
There is disclosed a light FSK (Frequency Shift Keying) modulator in which sub-Mach-Zehnder optical waveguides are each provided in two arms of a main Mach-Zehnder optical waveguide and in which an RF modulation process is performed in the sub-Mach-Zehnder optical waveguides to generate sideband beams (sidebands on the upper and lower sides) in the upper and lower portions of a frequency and a phase is selected in association with a data signal in the main Mach-Zehnder optical waveguide to output the sideband beams as signal beams subjected to a frequency modulation process by switching the sidebands to the upper and lower sides to output (for example, see Patent Document 1). In addition, there has recently been suggested a beam intensity modulator capable of realizing a high extinction ratio by using the sub-Mach-Zehnder optical waveguides as a light quantity adjusting unit and operating the light FSK modulator as the above-described beam intensity modulator (for example, see Non-patent Document 1).
Patent Document 1: Unexamined Japanese Patent Application, First Publication No. 2005-134897
Non-patent Document 1: Hikuma et al. “Wavelength Property of High Extinction Ratio Modulator to which Light FSK Modulator is Applied in 2005 Conference Journal of Institute of Electronic, Information, Communication Engineers Society, September 2005, c-3-2”